Method and apparatus for amplifying a signal and test device using same

Abstract

An amplifier circuit is used in a multimeter to amplify signals applied between a pair of test terminals. A voltage applied to one of the test terminals is amplified by a first operational amplifier configured as a voltage follower. An output of the first operational amplifier is applied to an inverting input of a second operational amplifier configured as an integrator. An output of the second operational amplifier is connected to the other of the test terminals. A voltage generated at the output of the second operational amplifier provides an indication of the magnitude and polarity of the voltage applied to the first and second test terminals.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 11/881,699, filed Jul. 26, 2007. This application is incorporated by reference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates generally to amplifiers for increasing the magnitude of a voltage, and, more particularly, to an amplifier and method having a high input impedance, high bandwidth and high signal-to-noise ratio.

BACKGROUND OF THE INVENTION

Devices for measuring various electrical parameters, such as voltage, current and resistance are in common use. A typical example is a multimeter, which generally can measure AC or direct current (“DC”) voltage and current as well as resistance. Multimeters typically include a set of test leads that are adapted to be connected to a pair of test points. The test leads are coupled to an internal amplifier, which drives circuitry for providing information to a read-out device such as an analog meter or a digital display. A typical amplifier circuit 10 is shown in FIG. 1. The amplifier circuit 10 includes an operational amplifier 12 having a non-inverting input connected to ground. An inverting input of the operational amplifier 12 forms a summing junction that is connected to one input terminal 14 of the multimeter through an input resistor 18 and to an output of the operational amplifier 12 through a feedback resistor 20. Another input terminal 24 is connected to ground. As is well-known in the art, the gain of the amplifier 12 is set by the ratio of the resistance of the feedback resistor 20 to the resistance of the input resistor 18.

It is generally desirable for a multimeter to have a very high input impedance. For this reason, the input resistor 18 typically has a very high resistance, such as 1 MΩ. The resistance of the feedback resistor 20 is typically much lower, such as 10 kΩ. Therefore, the gain of the amplifier 12 is low. Using the examples given (1 MΩ input resistor 18 and 10 kΩ feedback resistor 20), the gain of the amplifier 12 would be 0.01.

The output of the amplifier 12 is then applied to a high gain amplifier 30. The low gain of the amplifier 12 attenuates the signal to be measured, but, unfortunately, it does not significantly attenuate noise that may be present in the signal or present in the multimeter. Therefore, when the output of the amplifier 12 is boosted by the high gain amplifier 30, the signal-to-noise ratio of the measured signal can be very low.

Another “front end” amplifier circuit 40 that is conventionally used in multimeters is shown in FIG. 2. The circuit 40 also uses an operational amplifier 44 configured as a voltage-follower with its output connected to the inverting input of the amplifier 44. The signal to be measured is applied to the non-inverting input of the amplifier 44 through a resistor 46. The amplifier 44 has a very high input impedance between its inverting and non-inverting inputs. However, to fix the input impedance at a constant, controllable value, an input resistor 48 may be used between the input terminal 14 and ground. The input resistor 48 may have a high resistance value, such as 1 MΩ. Unfortunately, stray capacitance and input capacitance of the amplifier 44, both of which are represented by a capacitor C, forms a low-pass filter with the resistor 46. This low-pass filter can limit the AC response of the amplifier circuit 40. Although the circuit 40 can still be used in the measurement of the voltage and current of DC signals, the low-pass filter can result in measurement errors for AC signals, particularly if the AC signals have a high frequency.

There is therefore a need for a circuit for amplifying a signal to be measured in a manner that results in a high signal-to-noise ratio, a high, stable input impedance and good high frequency performance.

SUMMARY OF THE INVENTION

An apparatus and method for amplifying a signal applied between first and second input terminals includes an output voltage generator that provides an output voltage having a magnitude equal to the voltage applied to the first input terminal. The output voltage generator also provides the output voltage with a polarity that is opposite the polarity of the voltage applied to the first input terminal referenced to a fixed voltage. The output voltage generator provides the output voltage to the second input terminal. The output voltage generator may be a negative integrating driver circuit having a first input coupled to the first test terminal, a second input coupled to a fixed voltage, and an output coupled to the second test terminal. The negative integrating driver circuit is operable to integrate a voltage applied to the first test terminal. The integration accomplished by the negative integrating driver circuit is at a polarity opposite the polarity of the voltage applied to the first test terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a prior art amplifier circuit that is often used in a test and measurement device.

FIG. 2 is another embodiment of a prior art amplifier circuit that is often used in a test and measurement device.

FIG. 3 is an embodiment of an amplifier circuit according to one embodiment of the invention.

FIG. 4 is an embodiment of an amplifier circuit according to another embodiment of the invention.

FIG. 5 is a plan view of a multimeter shown measuring an AC voltage using the amplifier circuit of FIG. 3 or 4 or an amplifier circuit according to some other embodiment of the invention.

DETAILED DESCRIPTION

An amplifier circuit 50 according to one embodiment of the invention is shown in FIG. 3. The amplifier circuit 50 includes a negative integrating driver circuit 51 having a pair of input terminals 52, 53. One of the input terminals 52 is connected to a first test terminal 54, and the other input terminal 53 is connected to ground. An input voltage V_IN to be measured is applied between the first test terminal 54 and a second test terminal 56. The output of the negative integrating driver circuit 51 is connected to the second test terminal 56 and to a first output terminal 58. A second output terminal 59 is connected to ground. An output voltage V_OUT generated between the terminals 58, 59 provides an indication of the magnitude the voltage V_IN applied to the test terminals 54, 56. The negative integrating driver circuit 51 integrates with respect to time the input voltage V_IN applied between the test terminals 54, 56 at a polarity that is opposite the polarity of the input voltage V_IN. For example, if the input voltage V_IN is a constant +5 volts, the output of the negative integrating driver circuit 51 will increase negatively at a constant rate.

The amplifier circuit 50 has the unusual property of having the output of a circuit, i.e., the negative integrating driver circuit 51, connected to an input terminal, i.e., the test terminal 56. In operation, assume the voltage at terminal 56 referenced to ground is initially 0 volts. The voltage at terminal 54 referenced to ground will therefore be equal to the voltage V_IN applied between the test terminals 54, 56. This voltage is applied to the negative integrating driver circuit 51, which integrates this voltage negatively. Eventually, the voltage at the output of the negative integrating driver circuit 51 is equal to the −V_IN, i.e., the negative of the voltage V_IN between the terminals 54, 56. At this point, the voltage of the first test terminal 54 will be 0 volts, which is applied to the input of the negative integrating driver circuit 51. The negative integrating driver circuit 51 therefore stops integrating to maintain the voltage at the test terminal 56 at the negative of the voltage V_IN between the terminals 54, 56. The output voltage V_OUT taken between the terminals 58, 59 then has a value that is the inverse of the voltage V_IN being measured. For example, if +5 volts is applied between the terminals 54, 56, the output of the negative integrating driver circuit 51 will be −5 volts. At this point, the voltage at the terminal 54 referenced to ground will be 0 volts. As a result, the negative integrating driver circuit 51 will stop further integrating so that the voltage V_OUT between the output terminals 58, 59 will be maintained at −5 volts. The negative integrating driver circuit 51 can have an integration time constant that is short enough to provide the amplifier circuit with adequate high-frequency response. Thus, changes in the voltage applied between the test terminals 54, 56 that are within the frequency response of the negative integrating driver circuit 51 do not result in any current flow between the input terminals 54, 56. Therefore, the low frequency input impedance, i.e., de(t)/di(t), at the input terminals 54, 56 is virtually infinite as long as the isolation between either of the input terminals 54, 56 and ground is complete.

An amplifier circuit 60 according to another embodiment of the invention is shown in FIG. 4. The amplifier circuit 60 includes a negative integrating driver circuit 62, which includes an operational amplifier 64 configured as a voltage-follower by coupling its output to its inverting input. The non-inverting input of the amplifier 64 is connected to a first test terminal 66 through a resistor 70, which may have a value of 100k or any other suitable value. The output of the amplifier 64 is connected through a resistor 74 to an inverting input of another operational amplifier 80. This amplifier 80 has its non-inverting input grounded and its inverting input connected to its output through a capacitor 84 so that it functions as an inverting integrator. The integration time constant is set by the product of the resistance of the resistor 74 and the capacitance of the capacitor 84. In one embodiment, the resistor 74 has a resistance 200 ohms, and the capacitor has a capacitance of 100 picofarads, but other values can be used. By using the operation amplifier 64 as a voltage follower, the integration time of the amplifier 80 can be made proportional to the generally lower resistance of the resistor 74 rather than to the generally higher resistance of the resistor 70. The output of the amplifier 80 is connected to a second test terminal 88. A voltage V_IN to be measured is applied between the terminals 66, 88. A resistor 90 is connected between the test terminals 66, 88 to set the input impedance of the amplifier circuit 60. In one embodiment, the resistor 90 has a resistance 1 MΩ), but other values can be used. Also, the resistor 90 can be omitted to provide a virtually infinite low frequency input impedance. A first output terminal 94 of the amplifier circuit 60 is obtained from the output of the amplifier 80, and a second output terminal 96 is connected to ground. An output voltage V_OUT is generated between the terminals 94, 96.

In operation, the voltage V_IN applied between the test terminals 66, 88 is again applied to the amplifier 80. The amplifier 80 then integrates this voltage negatively. Eventually, the voltage at the output of the amplifier 80 referenced to ground is equal to the voltage V_IN between the terminals 66, 88. At this point, the voltage of the first test terminal 66 referenced to ground will be 0 volts, which is applied to the input of the amplifier 80. The amplifier 80 therefore stops integrating to maintain the voltage at the output of the amplifier 80 at the negative of whatever voltage V_IN is being measured between the terminals. The output voltage V_OUT taken between the terminals 94, 96 then has a value that is the inverse of the voltage V_IN being measured. For example, if +5 volts is applied between the terminals 66, 88, the output of the amplifier 64 will be at 5 volts. The amplifier 80 then begins integrating negatively so that the voltage at the output of the amplifier 80 negatively increases, thereby correspondingly reducing the voltage at the test terminal 66 toward 0 volts. When the integration has proceeded to the point that the output of the amplifier 80 is at −5 volts, the voltage at the terminal 66 will be 0 volts. At this point, the amplifier 64 will apply 0 volts to the amplifier 80, which will then stop further integrating so that the voltage V_OUT between the output terminals 94, 96 will be maintained at −5 volts referenced to ground. If the test terminals 66, 88 were suddenly shorted, a voltage of −5 volts would be applied to the amplifier 64, which would cause the amplifier 80 to integrate positively toward 0 volts. When the output of the amplifier 80 reached 0 volts, the integration would stop. By appropriately choosing the values of the resistor 74 and the capacitor 84, the integration time can be made sufficiently short that the high-frequency response of the amplifier circuit 60 is adequate.

The amplifier circuit 50 or 60, or an amplifier circuit according to some other embodiment of the invention, is shown used in a multimeter 100 in FIG. 5. The multimeter 100 includes a digital display 104, manually operable buttons 106, and a rotatable mode selector switch 108, which is shown in a position to measure AC voltage. Although the multimeter 100 uses a digital display 104, it will be understood that other types of displays may be used, such as an analog meter (not shown). Also, of course, controls other than the buttons 106 and selector switch 108, may be used. A pair of test leads 110, 112 have plugs 116, 118 that are plugged into respective jacks 120, 122. The plug 118 is inserted into the jack 122 to measure AC or DC voltage, but it is plugged into either of an additional pair of jacks 124, 126 to measure current or resistance, respectively. The plug 116 is inserted into the jack 120 for all measurements. The test leads 110, 112 are connected through respective test probes 130, 132 to respective test points in the form of respective wires 136, 138. The jack 120 is connected to an output terminal of a negative integrating driver circuit 140, which may be the negative integrating driver circuit 51 or 62, or a negative integrating driver circuit according to another embodiment of the invention. The jack 122 is connected to the input of the negative integrating driver circuit 140. An output of the negative integrating driver circuit 140 is connected to appropriate processing circuitry 144, which provides signals to the display 104 to provide a voltage indication.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the operational amplifier 64 may be omitted so that the test terminal 66 is applied directly to the amplifier 80 through one of the resistors 70 or 74. Other variations will be apparent to one skilled in the art. Accordingly, the invention is not limited except as by the appended claims.

Claims

1. An amplifier circuit, comprising:

a first test terminal;

a second test terminal;

a first output terminal;

a second output terminal coupled to a first fixed voltage; and

an output voltage generator having a first input coupled to the first test terminal, a second input coupled to a second fixed voltage, and an output coupled to the second test terminal and the first output terminal, the first and second test terminals being configured to receive an input voltage applied between the first and second test terminals, the output voltage generator being configured to apply to the second test terminal and the first output terminal a voltage corresponding to the magnitude of the second fixed voltage minus the magnitude of the input voltage.

2. The amplifier circuit of claim 1, further comprising an input resistor coupled between the first test terminal and the second test terminal.

3. The amplifier circuit of claim 1 wherein the first fixed voltage has a magnitude that is equal to the magnitude of the second fixed voltage.

4. The amplifier circuit of claim 3 wherein the first fixed voltage and the second fixed voltage are zero volts.

5. The amplifier circuit of claim 1 wherein the output voltage generator comprises a negative integrating driver circuit having a first input coupled to the first test terminal, a second input coupled to the second fixed voltage, and an output coupled to the second test terminal, the negative integrating driver circuit being configured to integrate a voltage applied to the first test terminal referenced to the second fixed voltage, the integration being at a polarity opposite the polarity of the voltage applied to the first test terminal referenced to the second fixed voltage, the negative integrating driver circuit providing an output voltage to the output of the negative integrating driver circuit that is indicative of the integration.

6. The amplifier circuit of claim 5 wherein the negative integrating driver circuit comprises:

an amplifier having a gain, the amplifier having a first input coupled to the first test terminal, a second input coupled to the second fixed voltage, and an output; and

an integrator having an input coupled to the output of the amplifier and an output coupled to the second test terminal, the integrator being configured to integrate a voltage received from the output of the amplifier to provide an output voltage to the second test terminal, the integrator having an integration polarity that is opposite a polarity of the gain of the amplifier.

7. The amplifier circuit of claim 6 wherein the amplifier has a unity gain.

8. The amplifier circuit of claim 7 wherein the amplifier comprises an operational amplifier having an inverting input connected to an output of the operational amplifier, and a non-inverting input coupled to the first test terminal.

9. The amplifier circuit of claim 8 wherein the amplifier further comprises a resistor connected between the first test input terminal and the non-inverting input of the operational amplifier.

10. The amplifier circuit of claim 1 wherein the output voltage generator is configured to apply a voltage to the second test terminal that causes the voltage at the first test terminal to be equal to the second fixed voltage when the input voltage is being applied between the first and second test terminals.

11. A method of amplifying an input voltage, comprising:

applying the input voltage between first and second input terminals, the input voltage being equivalent to the potential difference between the first and second input terminals;

generating an output voltage on the basis of a comparison between the voltage at the first input terminal and a fixed voltage; and

applying the output voltage to the second input terminal, the output voltage applied to the second input terminal causing the voltage at the first input terminal to be equal to the fixed voltage when the input voltage is being applied between the first and second input terminals.

12. The method of claim 11 wherein the act of generating the output voltage comprises integrating an intermediate voltage with respect to time, the intermediate voltage having a magnitude determined by the magnitude of the voltage applied to the first input terminal referenced to the fixed voltage.

13. The method of claim 11 wherein the act of generating an output voltage comprises generating an output voltage having a magnitude determined by frequency components of the input voltage that are below predetermined frequency limit.